This invention relates to a microscope such as an Atomic Force Microscope (AFM), a Magnetic Field Microscope (MFM), etc. and particularly to a microscope for measuring a large-size sample.
In a conventional AFM apparatus, an optical lever system is disclosed as a displacement detection system in the z-direction (the thickness direction of a sample) in a paper entitled "An atomic-resolution atomic-force microscope implemented using an optical lever" by S. Alexander, et al., published in the Journal of Applied Physics 65 (1), 164-167 (1988). Further, an optical wave interference system is also disclosed in "Atomic Force microscope-force mapping and profiling on a sub 100 .ANG. scale" by Y. Martin, et al., Journal of Applied Physics 61 (10), 1987. In both systems, a force occurring between a sample surface and a probe having a sharp tip is converted to the displacement of a cantilever having the shape of a weak spring, the probe being secured to the tip of the cantilever.
In the first method, an opposite-side surface to the probe of the spring-shaped cantilever is mirror polished, a laser light is irradiated onto the mirror polished surface, and light reflected therefrom is detected by a bisectional photodiode. Thereafter, the difference in light amount detected by the two photodiode parts is detected as the displacement of the cantilever (spring), that is, an uneven status of the sample surface, and then converted to an electrical signal.
In the second method, the spring is vibrated at its resonant frequency, and its frequency and amplitude are monitored by an optical wave interference method. The frequency and amplitude of this vibration is varied due to a fine force acting between the sample and the probe and this variation is detected as an electrical signal (hereinafter referred to as "AC detection method").
The force acting between the sample and the probe is converted to a signal representing the displacement of the spring, and the converted electrical signal is input to a servo system and thus a z direction piezoelectric scanner at a sample stand side to thereby control the z-direction of the sample stand so that the force acting between the sample and the probe is constant. At this time, an in-plane scanning of the probe is carried out by the XY direction piezoelectric scanner in a fixed area of the sample, and through this operation a three-dimensional image of the sample surface is obtained.
At present, in the optical lever system, the sample surface is scanned with a force of about 10.sup.-9 N acting between the sample and the probe by a direct-current detection method, and it has been widely used for AFM. On the other hand, the AC detection method has been exclusively used in the optical wave interference system, and it has been used as a spring displacement detection system for a magnetic force microscope.
Hitherto, in both the optical lever system and the optical wave interference system, the sample is mounted on the three-dimensional piezoelectric scanner (scanner tube), and the probe is relatively swept on the sample surface at high sped. In order to carry out a high-speed scanning, it is required to miniaturize the scanner and keep it highly rigid. Therefore, the construction that the sample is mounted on the three-dimensional scanner tube imposes restrictions on the size and weight of the sample. At present, the size and weight of a securable sample is about 2 cm square and about 1 g, respectively.
A second problem will occur in complex technologies with other observing means. In the optical lever system, a fixed positional relationship is required to be kept between a laser generator serving as a light source, a cantilever and a photodiode, and these elements cover most of the sample surface to be measured and observed. Therefore, it is difficult to install an optical lever system of an AFM apparatus into a revolver portion for exchanging an objective lens of an optical microscope.
In the optical wave interference system, an interferometer portion can be disposed far away and a detection portion can be also disposed far away from the sample because an optical fiber is used. Therefore, it is effectively used for composite with other apparatuses and remote detection. However, in order to stably obtain interference fringes, a stable He-Ne laser must be used, and thus the whole construction of the apparatus is required to be designed to be of large size.